Interpretive Summary: Zinc (Zn) is an essential plant micronutrient that often limits crop production as the soils on over 30% of the world’s arable lands are Zn deficient. Also, it is estimated that 2 billion people worldwide suffer from Zn deficiency. Therefore, research on understanding how plants acquire and accumulate this sparingly available micronutrient can provide the necessary information to improve both plant and human Zn nutrition. In this manuscript, we describe and detail research on a unique plant species, Noccaea caerulescens, that accumulates Zn to astoundingly high levels in its leaves (100x the leaf Zn concentrations in normal plants). We are using this plant as a model system to study how plants can very efficiently and effectively acquire Zn. We identified the first metal transporter gene in a hyperaccumulator that we named NcZNT for zinc transporter in Noccaea. It was identified because the gene is very highly expressed in this plant. We further found that the transporter is very active in roots and shoots of Noccaea, and mediates Zn uptake into specific cells in the root and shoot involved in long distance transport of Zn. We found that NcZNT1 plays a key role in moving the large amount of Zn absorbed by this hyperaccumulator from the root to the shoot where it is stored in the leaves. These findings are enabling us to begin to understand how plants regulate at the molecular level, the network of proteins involved in Zn uptake from the soil, and Zn accumulation in the portions of the plant consumed by humans. Thus these findings have implications for dealing with both plant and human Zn deficiency.

Technical Abstract:
In this paper, we conducted a detailed analysis of the ZIP family transporter, NcZNT1, in the Zn/Cd hyperaccumulating plant species, Noccaea caerulescens, formerly known as Thlaspi caerulescens. NcZNT1 was previously suggested to be the primary root Zn/Cd uptake transporter. Both a characterization of NcZNT1 transport function in planta and in heterologous systems, as well as analysis of NcZNT1 gene expression and NcZNT1 protein localization was carried out. We show that NcZNT1 is not only expressed in the root epidermis, but also is highly expressed in the root and shoot vasculature, suggesting a role in long distance metal transport. Also, NcZNT1 was found to be a plasma membrane transporter that mediates Zn but not Cd, Fe, Mn or Cu uptake into plant cells. Two novel regions of the NcZNT1 promoter were identified which may be involved in both the hyperexpression of NcZNT1 and its ability to be regulated by plant Zn status. In conclusion, we demonstrate here that NcZNT1 plays a role in Zn and not Cd uptake from the soil, and based on its strong expression in the root and shoot vasculature, could be involved in long distance transport of Zn from the root to the shoot via the xylem.